What Happens in a Stroke (and Why Minutes Matter)
A stroke is a brain attack: brain tissue dying because its blood supply is suddenly cut off — or bleeding. Press play and watch a clot fly out of a fibrillating heart, jam a brain artery, and kill a core of tissue in minutes. But look at the amber rim around it: the penumbra is stunned, not yet dead — and it can still be saved if blood returns fast. That is the whole reason the ambulance runs red lights. An untreated large stroke destroys roughly 1.9 million neurons every minute.
Try this: leave it on Ischemic, let the clot lodge, then hit 💉 Give tPA / thrombectomy early and watch the penumbra turn pink again. Then switch to Hemorrhagic and press the same button — and see why a CT scan must come first.
Live stroke readout
What's happening
Real numbers: ~85% of strokes are ischemic and ~15% hemorrhagic; normal cerebral blood flow is ~50 mL/100g/min, the penumbra survives around 10–20 and the core below ~10; the ~1.9 million neurons/minute figure is Saver’s published average for a typical large-vessel ischemic stroke; the clot-buster (tPA) window is ~4.5 hours and thrombectomy can help selected patients up to 24 hours. Illustrative: the dot pattern, the exact core-growth speed, and the on-screen clock rate are a simplified model, not a measurement of any one patient.
The Science in Plain Language
Two very different emergencies hide behind one word
“Stroke” actually means two opposite disasters. In an ischemic stroke (about 85% of cases) a blood clot plugs a brain artery and the tissue downstream starves. In a hemorrhagic stroke (about 15%) a vessel bursts and leaking blood both starves the tissue beyond the break and crushes the tissue around it. They look almost identical at the bedside — sudden weakness, drooping face, garbled speech — which is exactly why the treatments cannot be guessed. The drug that saves the first kind can kill someone with the second. That single fact drives everything else on this page.
Where the clot comes from
Most ischemic strokes are embolic or thrombotic. An embolus is a clot that formed somewhere else and sailed up into the brain. The classic source is the heart in atrial fibrillation (AFib): the top chambers quiver instead of squeezing, blood pools and clots in a pouch called the left atrial appendage, and a piece breaks loose. AFib raises stroke risk roughly five-fold and is behind a large share of the most disabling strokes. Others start locally, on a ruptured cholesterol plaque in a neck or brain artery — the same plaque story told on the cholesterol page, and the same clot chemistry shown in how blood clots.
The idea that saves lives: core versus penumbra
When flow stops, the tissue does not all die at once. Dead center is the core, where flow has crashed below about 10 mL/100g/min (normal is ~50) — those neurons are gone within minutes. But neighbouring arteries leak a trickle of blood sideways into the rim, keeping it at roughly 10–20 mL/100g/min. That rim is the penumbra: cells that have switched themselves off to survive — electrically silent, so the patient has symptoms — but still structurally alive. The penumbra is the tissue medicine is fighting for. Restore flow and it wakes back up. Wait too long and it quietly converts into more core. Watch the amber ring in the animation shrink into gray: that is a person losing an arm, a word, a memory.
“Time is brain” — the number that runs the ambulance
Neurologist Jeffrey Saver put arithmetic to the slogan. In a typical large-vessel ischemic stroke, the untreated brain loses about 1.9 million neurons every minute — roughly 14 billion synapses and 12 km of nerve fibers per minute, or 120 million neurons per hour. Put another way, each hour without treatment ages the affected brain by about 3.6 years. These are averages, not a stopwatch on your exact cells, but the direction is brutal and real: every minute of delay costs tissue you can never regrow. That is why calling an ambulance beats driving yourself, and why the clock starts at the last time you were known to be well.
Reading the map: FAST and BE-FAST
Symptoms point straight at the injured territory. The middle cerebral artery feeds the strip of brain that runs the face, arm and speech, so the everyday warning sign is FAST: Face drooping on one side, Arm weakness or drift, Speech slurred or wrong, Time to call emergency services now. The newer BE-FAST adds Balance (sudden dizziness, falling) and Eyes (sudden vision loss or double vision), which catch strokes in the back of the brain that plain FAST can miss. Any one of these, arriving suddenly, is a stroke until proven otherwise.
The treatment race: bust it or pull it out
For an ischemic stroke there are two ways to reopen the artery. Clot-busting drugs — tPA (alteplase), and increasingly tenecteplase — are given through a vein within about 4.5 hours of onset; they dissolve the clot the same way the body’s own system does in how blood clots dissolve. For big clots in large arteries, mechanical thrombectomy threads a catheter up from the groin and physically drags the clot out — and thanks to trials like DAWN and DEFUSE-3, that can help carefully selected patients out to 24 hours when imaging shows a penumbra still worth saving. Sooner is always better: reopen early and much of the penumbra survives.
Why the CT scan cannot be skipped
Here is the hinge of the whole emergency. tPA is a clot-dissolver. Give it to someone whose problem is a burst vessel and you dissolve the very clot plugging the leak — the bleed explodes, often fatally. So before any clot-buster, an emergency non-contrast CT scan of the head is done in minutes to answer one question: clot or bleed? On CT, fresh blood shows up bright white; a pure ischemic stroke usually looks near-normal early on. Only once bleeding is ruled out does the clot-buster go in. In the animation, press Give tPA during the hemorrhagic scenario and you’ll see exactly why this step is non-negotiable.
Two kinds of bleed
The hemorrhagic side splits in two, and the split matters. An intracerebral hemorrhage (ICH) is bleeding into the brain tissue itself, most often from small vessels worn out by years of high blood pressure — which is why controlling blood pressure is the single biggest thing that prevents it. A subarachnoid hemorrhage (SAH) is bleeding into the thin fluid space wrapping the brain, usually from a ruptured aneurysm (a balloon on an artery wall). Its signature is the “thunderclap” headache — the worst headache of your life, arriving in seconds — sometimes with a stiff neck and vomiting. A sudden, explosive headache like that is its own reason to call emergency services immediately, headache being the one stroke symptom the FAST test does not cover. Both bleeds share the rule you just watched: no clot-buster, ever, until a scan has ruled bleeding out.
An honest myth to correct — and how to prevent the next one
The dangerous myth: “It’ll pass, let’s wait and see.” A stroke rarely hurts, so people lie down and hope. And sometimes the symptoms do vanish in an hour — that is a transient ischemic attack (TIA), a clot that cleared on its own. A TIA is not a lucky break; it is a fire alarm. The risk of a full, permanent stroke in the days right after a TIA is high, and it is one of the few times where getting checked immediately genuinely changes the outcome. Never wait out sudden weakness, drooping, or speech trouble. And most strokes are preventable: control blood pressure (the single biggest lever, and the main cause of hemorrhagic strokes), treat AFib with anticoagulants that cut its stroke risk by roughly two-thirds, lower cholesterol, manage diabetes, and above all stop smoking. Learn the fuller picture on the Stroke page.